Like ladies' clothing, it's not so much the cut or the style as it is the fabric and the elegant embellishments that set fine garments apart from workaday middling sorts' clothes. Workaday middling sorts such as merchants, physicians, better-off farmers all would have had well-tailored three-piece suits. They just wouldn't have been made of kicky patterned silk or embroidered to high heaven.
This ensemble is made of matching silk coat and breeches with an ivory weskit, embroidered in a pattern to match the coat. The Met Museum Costume Institute dates it to 1774-1792, but I'm not sure that this is accurate--the breeches don't have the characteristic fall-front from that period, and the larger cuffs with big buttons, and the largeish decorative pocket flaps have the air of an earlier decade--I'd tentatively say 1760s, but what do I know? Perhaps this piece is a "transition" from the more ostentatious midcentury to the more demure late century look--or was an older man's, and he had no interest in keeping up with trends.
One of my favorite details--each button on the weskit is covered with matching embroidery. Little pink flowers...when did men decide they couldn't or wouldn't wear clothing embroidered with little pink flowers? I think the effect here is quite fetching.
One note--though this may or may not be the case for this particular piece, intricately emroidered weskits like this one were often created in an interesting way. Workshops dedicated to embroidery would create the complicated designs, usually worked in fine silk crewelwork. However, they weren't working on a fully assembled piece--they embroidered the cut pieces and then shipped those off to tailors, who fit the garments to a particular client.